CHAPTER ELEVEN COMMUNICATING ENTHALPY CHANGES

Today’s Agenda: Give homework solutions.  Communicating Enthalpy PowerPoint Curricular outcomes: 1.3k: Define enthalpy and molar enthalpy for chemical reactions 1.4k: Write balanced equations for chemical reactions that include energy changes 1.5k: Use and interpret ΔH notation to communicate and calculate energy changes in chemical reactions

C OMMUNICATING E NTHALPY We will be learning how to communicate enthalpy changes in four ways: 1. By stating the molar enthalpy of a specific reactant in a reaction 2. By stating the enthalpy change for a balanced reaction equation 3. By including an energy value as a term in a balanced reaction equation 4. By drawing a chemical potential energy diagram

COMMUNICATING ENTHALPY #1 1. By stating the molar enthalpy of a specific reactant in a reaction Why do we use standard conditions in chemistry (i.e. SATP)? We use a standard set of conditions so that scientists can create tables of precise, standard values and can compare other values easily Do we have standard conditions for enthalpy?? Yes, we will be using SATP (but liquid and solid compounds must only have the same initial and final temperature – most often 25°C) How do we communicate that standard conditions are used for reactants and products? With a ° superscript, such as Δ f H m ° or Δ c H m ° (See data booklet pg. 4 and 5) *For well-known reactions such as formation and combustion, no chemical equation is necessary, since they refer to specific reactions with the Δ f or Δ m ** Would the sign for Δ f H m ° be the opposite of the sign for Δ d H m ° (decomposition)? YES! *For equations that are not well known or obvious, then the chemical equation must be stated along with the molar enthalpy.

COMMUNICATING ENTHALPY #1 1. By stating the molar enthalpy of a specific reactant in a reaction Example #1: This means that the complete combustion of 1 mol of methanol releases releases kJ of energy according to the following balanced equation Example #2: This does not specify a reaction, so a chemical equation must be stated along with the molar enthalpy. This is not a formation reaction, since not all of the reactants are elements, so this could not have been communicated with Δ f

COMMUNICATING ENTHALPY #2 2. By stating the enthalpy change beside a balanced reaction equation Do we know how to calculate enthalpy change?? The enthalpy change for a reaction can be determined by multiplying the chemical amount (from the coefficient in the equation) by the molar enthalpy of reaction (for a specific chemical) Example: Sulfur dioxide and oxygen react to form sulfur trioxide. The standard molar enthalpy of combustion of sulfur dioxide, in this reaction, is kJ/mol. What is the enthalpy change for this reaction? 1) Start with a balanced chemical equation. 2) Then determine the chemical amount of SO 2 from the equation = 2 mol (this is an exact #, don’t use for sig digs) 3) Then use to determine the enthalpy change for the whole reaction. 4) Then report the enthalpy change by writing it next to the balanced equation.

COMMUNICATING ENTHALPY #2 2. By stating the enthalpy change beside a balanced reaction equation THE ENTHALPY CHANGE DEPENDS ON THE ACTUAL CHEMICAL AMOUNT OF REACTANTS AND PRODUCTS IN THE CHEMICAL REACTION. THEREFORE, IF THE BALANCED EQUATION IS WRITTEN DIFFERENTLY, THE ENTHALPY CHANGE SHOULD BE REPORTED DIFFERENTLY Both chemical reactions agree with the empirically determined molar enthalpy of combustion for sulfur dioxide

COMMUNICATING ENTHALPY #2 2. By stating the enthalpy change beside a balanced reaction equation THE ENTHALPY CHANGE DEPENDS ON THE ACTUAL CHEMICAL AMOUNT OF REACTANTS AND PRODUCTS IN THE CHEMICAL REACTION. THEREFORE, IF THE BALANCED EQUATION IS WRITTEN DIFFERENTLY, THE ENTHALPY CHANGE SHOULD BE REPORTED DIFFERENTLY Example 2: 2Al (s) + 3Cl 2(g)  2AlCl 3(s) Δ f H° = kJ What is the molar enthalpy of formation of aluminum chloride? Δ f H m ° = kJ = kJ/mol AlCl 3 2 mol

COMMUNICATING ENTHALPY #2 2. By stating the enthalpy change beside a balanced reaction equation EXAMPLE: The standard molar enthalpy of combustion of hydrogen sulfide is kJ/mol. Express this value as a standard enthalpy change for the following reaction equation: SOLUTION:

COMMUNICATING ENTHALPY #3 3. By including an energy value as a term in a balanced reaction equation If a reaction is endothermic, it requires additional energy to react, so is listed along with the reactants If a reaction is exothermic, energy is released as the reaction proceeds, and is listed along with the products In order to specify the initial and final conditions for measuring the enthalpy change of the reaction, the temperature and pressure may be specified at the end of the equation

COMMUNICATING ENTHALPY #3 3. By including an energy value as a term in a balanced reaction equation EXAMPLE: Ethane is cracked into ethene in world-scale quantities in Alberta. Communicate the enthalpy of reaction as a term in the equation representing the cracking reaction. D OES THE kJ MEAN EXOTHERMIC OR ENDOTHERMIC ?

COMMUNICATING ENTHALPY #3 3. By including an energy value as a term in a balanced reaction equation EXAMPLE: Write the thermochemical equation for the formation of 2 moles of methanol from its elements if the molar enthalpy of formation is kJ/mol 2 C (s) + 4 H 2(g) + O 2(g)  2 CH 3 OH (l) + ___?_____ Δ f H = 2 mol ( kJ/mol) = kJ (Exothermic) 2 C (s) + 4 H 2(g) + O 2(g)  2 CH 3 OH (l) kJ

COMMUNICATING ENTHALPY #4 4. By drawing a chemical potential energy diagram During a chemical reaction, observed energy changes are due to changes in chemical potential energy that occur during a reaction. This energy is a stored form of energy that is related to the relative positions of particles and the strengths of the bonds between them. As bonds break and re-form and the positions of atoms are altered, changes in potential energy occur. Evidence of a change in enthalpy of a chemical system is provided by a temperature change of the surroundings. A chemical potential energy diagram shows the potential energy of both the reactants and products of a chemical reaction. The difference is the enthalpy change (obtained from calorimetry) Guidelines: The vertical axis represents E p. The reactants are written on the left, products on the right, and the horizontal axis is called the reaction coordinate or reaction progress.

COMMUNICATING ENTHALPY #4 During an exothermic reaction, the enthalpy of the system decreases and heat flows into the surroundings. We observe a temperature increase in the surroundings. During an endothermic reaction, heat flows from the surroundings into the chemical system. We observe a temperature decrease in the surroundings.

COMMUNICATING ENTHALPY #4

EXAMPLE: Communicate the following enthalpies of reaction as a chemical potential energy diagram. The burning of magnesium to produce a very bright emergency flare. The decomposition of water by electrical energy from a solar cell.

Today’s homework: Read Section 11.3 Worksheet Extra Practice if you feel you need it Pg. 501 #1, 2a), 3, 4, 5 Curricular outcomes: 1.3k: Define enthalpy and molar enthalpy for chemical reactions 1.4k: Write balanced equations for chemical reactions that include energy changes 1.5k: Use and interpret ΔH notation to communicate and calculate energy changes in chemical reactions